Chronic hypoxia-induced acid-sensitive ion channel expression in chemoafferent neurons contributes to chemoreceptor hypersensitivity

Abstract

Previously we demonstrated that chronic hypoxia (CH) induces an inflammatory condition characterized by immune cell invasion and increased expression of inflammatory cytokines in rat carotid body. It is well established that chronic inflammatory pain induces the expression of acid-sensitive ion channels (ASIC) in primary sensory neurons, where they contribute to hyperalgesia and allodynia. The present study examines the effect of CH on ASIC expression in petrosal ganglion (PG), which contains chemoafferent neurons that innervate oxygen-sensitive type I cells in the carotid body. Five isoforms of ASIC transcript were increased ∼1.5–2.5-fold in PG following exposure of rats to 1, 3, or 7 days of hypobaric hypoxia (380 Torr). ASIC transcript was not increased in the sympathetic superior cervical ganglion (SCG). In the PG, CH also increased the expression of channel-interacting PDZ domain protein, a scaffolding protein known to enhance the surface expression and the low pH-induced current density mediated by ASIC3. Western immunoblot analysis showed that CH elevated ASIC3 protein in PG, but not in SCG or the (sensory) nodose ganglion. ASIC3 transcript was likewise elevated in PG neurons cultured in the presence of inflammatory cytokines. Increased ASIC expression was blocked in CH rats concurrently treated with the nonsteroidal anti-inflammatory drug ibuprofen (4 mg·kg−1·day−1). Electrophysiological recording of carotid sinus nerve (CSN) activity in vitro showed that the specific ASIC antagonist A-317567 (100 μM) did not significantly alter hypoxia-evoked activity in normal preparations but blocked ∼50% of the hypoxic response following CH. Likewise, a high concentration of ibuprofen, which is known to block ASIC1a, reduced hypoxia-evoked CSN activity by ∼50% in CH preparations. Our findings indicate that CH induces inflammation-dependent phenotypic adjustments in chemoafferent neurons. Following CH, ASIC are important participants in chemotransmission between type I cells and chemoafferent nerve terminals, and these proton-gated channels appear to enhance chemoreceptor sensitivity.